This invention relates to a replaceable or interchangeable finder system for a single-lens reflex (SLR) camera.
In the development of flexible SLR camera systems, the finders are commonly designed to be replaceable or interchangeable, whereby a variety of different finders (waist level finders and high magnification finders, for example) can be selectively used for specialized purposes.
Recently, a number of SLR cameras having automatic exposure control systems have also been developed. In such cameras it is convenient for the photographer to have the controlled exposure parameters (such as shutter speed when the aperture opening has priority) displayed in the finder, and such a display is employed in most of the newer automatic exposure control SLR cameras.
Heretofore, in order to display an exposure meter indication in a replaceable finder for an SLR camera, two methods have been employed: in one outside light (other than light passing through the lens) is introduced into the penta-prism through small prisms, while in the other a complete meter display, including an electrical drive circuit therefor, is included in each replaceable finder. The use of small prisms increases the cost of each finder, however, and in addition the viewing conditions of the finder are degraded by such prisms. In the latter method, on the other hand, a separate meter indicator for each finder greatly increases the cost thereof, and in addition a detachable power source connection between the camera body and the replaceable finder is required.
FIG. 5 shows a rear sectional view of a conventional finder system, wherein reference numeral 101 designates a camera body, 102 a top cover, 103 a focal plate, 104 a condenser lens, 107 a replaceable or interchangeable penta-prism, 108 a finder body, and 109 a cover for the finder body. The focal plate 103 and condenser lens 104 are combined into one unit by a frame 106 and a retaining plate 105. Thus, after the finder is removed from the camera body 101, the focal plate and condenser lens unit can also be removed in the same direction. To display the exposure meter indication in the finder it is necessary to form an image of the exposure meter therein using small prisms, or to mount an exposure meter on the side of each replaceable finder, as described above. FIG. 5 shows an example of the former case, wherein the images of a meter pointer 113, a manual pointer 112, and a shutter speed scale 111 are bent by a first small prism 110 with light entering the camera through a window 102a in the top cover 102 and brought into the penta-prism 107 through a second small prism 114 cemented thereto. To display the exposure meter indication in the finder the construction and configuration thereof must be such that the second small prism 114 can easily be adhered to it. This reduces the viewing conditions in the finder, and the cost of the camera is attendantly increased by the presence of the small prisms. The alternative of employing LEDs, on the other hand, makes the arrangement of the camera circuitry unduly intricate.